In a typical wireless communication system, the radio link between a user terminal (e.g., a cellular telephone) and the base station serving the user terminal is capable of conveying continuous voice signals in both directions, i.e., from the user terminal to the base station (and on to a remote party) and from the base station to the user terminal. However, in normal conversation, each person speaks on average for less than 50% of the total time. Transmitting radio signals that effectively carry nothing but silence creates unnecessary interference and wastes energy. Likewise, at the receiver end of the link, decoding radio signals carrying no voice signal wastes computing resources and also wastes energy. Accordingly, many wireless systems, including those based on the well-known Global System for Mobile Communications, support a Discontinuous Transmission (DTX) feature that allows the radio transmitter to be switched off during periods when there is no voice signal to be sent.
DTX is enabled by a family of techniques known as voice activity detection (VAD), also known as speech activity detection or speech detection, which are used to detect the presence or absence of human speech in a signal. DTX is a mechanism that allows the radio transmitter to be switched off during periods of no speech activity, to achieve power savings in the user terminal and overall system level interference reduction over the air interface. With DTX, no speech signal is transmitted during the absence of the speech detected by the VAD. This state is known as DTX state.
When the receiver at the other end of the radio link detects that the transmitter has not transmitted speech frames, it marks the frame as ‘bad’, which is known in GSM specifications as “bad frame indication” (BFI), and generates appropriate comfort noise, so that the user feels that the connection is still active. Generally, in the GSM case, when a transmitter enters the DTX state it transmits a Silence Interval Descriptor (SID) frame after the last speech frame; the SID frame is used by the receiver to generate comfort noise. The transmitter thereafter transmits one SID frame at regular intervals of 480 milliseconds, for so long as there is no voice activity. The signalling of the Slow Associated Control Channel (SACCH) remains active during the DTX period. When a GSM mobile station (MS) is in DTX mode, only twelve TDMA bursts are sent in each 480-millisecond interval; of those, four bursts are for a SACCH frame and eight are for SID frame transmission. Currently, a similar DTX functionality is implemented in almost all mobile communications standards (e.g., GSM, WCDMA, LTE), for the downlink (base-station-to-mobile-terminal) as well as in the uplink (mobile-terminal-to-base-station). Additional information about VAD and DTX can be found in the 3rd-Generation Partnership Project (3GPP) standards directed to specifying speech handling in 3GPP networks, which can be found at http://www.3gpp.org/ftp/Specs/html-info/06-series.htm.
During a DTX period, the receiver at the other end of the link receives only a noise signal, if it opens up a radio frequency (RF) “window” during the no-transmission period. Ideally, the receiver should not process this noise signal. However, DTX condition detection in a receiver is not always easy. Several problems arise if DTX is not reliably detected. For instance, if a speech frame is wrongly detected as silence then the speech carried by that frame will not be played, leading to a degrading of speech performance. On the other hand, if a silence period or no-transmission period is wrongly detected as speech then a noise signal is improperly converted into an audio signal, resulting in poor speech quality and the unnecessary consumption of extra power for processing the signal. Furthermore, Automatic Frequency Control (AFC) and Automatic Time Correction (ATC) values can be wrongly updated from a noise signal burst, which will cause deterioration of the frequency- and time-synchronization of the receiver.
Release 9 of the 3GPP GSM/EDGE Radio Access Network (GERAN) specifications introduces a new feature, called “Voice Services over Adaptive Multi-user channels on One Slot,” or VAMOS, which can potentially double the network voice channel capacity by assigning the same time slot and frequency for simultaneous use by two paired mobile stations. More about VAMOS can be found in the 3GPP report, “Circuit switched voice capacity evolution for GSM/EDGE Radio Access Network (GERAN),” 3GPP TR 45.914, v. 11.0.0 (September 2012), available at www.3gpp.org. Additional information can also be found in chapter 11 of Sajal Kumar Das, Mobile Handset Design, Wiley (2010).
In the downlink direction, the VAMOS solution introduces an Adaptive QPSK (AQPSK) modulation scheme, in contrast to the Gaussian Minimum-Shift Keying (GMSK) modulation used in prior systems. This enables the scheduling of two users on in-phase (I) and quadrature-phase (Q) subchannels, known as subchannel 1 and subchannel 2, respectively. Allocation of different power levels to each subchannel is also possible. So, one user or MS uses the I channel and the other uses the Q channel, for simultaneous transmission of voice using the same time slot and frequency. A parameter α, where 0≦α≦√2, is chosen to create a quaternary constellation—the resulting constellation points of the AQPSK modulator are: (±α/√2, ±j√(2−α2)/√2). FIG. 1 illustrates an example AQPSK constellation diagram. As seen in the diagram, each symbol contains two bits (one for each mobile station), so there are four points in the constellation. Because the power to the I and Q channels may be imbalanced, the deviations in the I and Q directions of the constellation may differ, as is the case in the example shown in FIG. 1. The ratio of power between the Q and I channels is defined as the Subchannel Power Imbalance Ratio (SCPIR). The value of the SCPIR is given by:SCPIR=20*log 10(tan(α))dB=Powersubchannel-2/Powersubchannel-1
According to 3GPP TS 45.001, “in downlink, for a VAMOS pair, when DTX is employed for the TCH channels, AQPSK modulation as shown above shall be used when on a given physical resource both the TCH channels in the VAMOS pair have bursts scheduled for transmission simultaneously. If only one of the TCH channels in a VAMOS pair has bursts scheduled for transmission, with the other TCH channel being in DTX state (having no bursts scheduled for transmission, see 3GPP TS 45.008), the BSS shall send GMSK normal bursts. If none of the TCH channels in the VAMOS pair has bursts scheduled for transmission, then nothing is transmitted.”
This excerpt from the 3GPP standards indicates that AQPSK modulation is only used if both users have bursts scheduled for transmission. If one of the paired VAMOS users is in DTX state, i.e., the voice transmission for one of the users is interrupted because of speech pauses, and then the base station (a BSS, in GSM terminology) will use traditional GMSK modulation instead of AQPSK. If neither user/subchannel is active then the network does not send anything, except SID updates and SACCH frame data for GSM. In this condition, both receivers must detect the DTX scenario properly and must also compute the subchannel power imbalance ratio (SCPIR) appropriately. Note that the measured ratio of powers can approach infinity in this scenario, as both powers are ideally zero. This can lead to an improper computation of the SCPIR, if proper care is not taken in the computational algorithm. That might lead to unexpected issues as the SCPIR value is used by the receiver for dynamic detection of interference cancellation, among other things.
The problem of reliably detecting a DTX condition is an old one. Prior patents and patent application publications have addressed this problem in several different ways. For example, US Patent Application Publication 2003/0142728 describes a two-dimensional quality metric, generated using an energy-per-bit-to-noise-power ratio as the first dimension and a re-encoded symbol error count as a second dimension, which is used to characterize the received frame as an erasure or as DTX. U.S. Pat. No. 6,904,557 describes a decoding of the received signal over a frame to determine a cumulative metric, which is then compared with a threshold. U.S. Pat. No. 7,616,712 describes Cyclic Redundancy Code (CRC) checking, whereby a received burst is classified as a normal burst or not based on a preliminary classification and the validity of CRC. Several other patents and patent application publications describe other techniques, including: US Patent Application Publication 2004/0095918; U.S. Pat. No. 6,725,054; U.S. Pat. No. 7,782,820; U.S. Pat. No. 5,936,979; U.S. Pat. No. 6,587,447; U.S. Pat. No. 6,038,238; U.S. Pat. No. 7,437,172; and U.S. Pat. No. 6,370,392.
All of the methods described in these references suffer from one or more issues, however. For instance, many of these methods are based on a metric that characterizes frame quality. To use these methods, the receiver has to receive all the bursts of that frame/block, then trigger the decoder, and then compute the frame quality based on one or more of several metrics, and then compare the results with a threshold. Some methods are also based on CRC checking. These techniques assume that if a frame is a DTX frame, then it will show up as a bad quality frame, or the CRC will fail, etc. But these techniques have several issues. First, the receiver has to receive all four bursts of the frame—only then can it decode and estimate block or frame quality. This leads to a significant time delay in detection. Further, unnecessary power and processing resources will be consumed due to the unnecessary reception and processing (including demodulation and block decoding) of all bursts of the invalid frame/block.
Other techniques are based on a bit-energy-to-noise ratio (Eb/N0) computation. This approach also has several issues. Again, these approaches may involve a delay in detection, as these techniques generally take all bursts of the frame and include decoding of the block. Further, techniques based only on Eb/N0 are not very reliable, yielding excessive false detections and/or missed detections. Methods based on received power measurements have similar issues.
DTX detection is complicated by the introduction of VAMOS. Previous DTX-detection techniques are not likely to be sufficient in VAMOS scenarios. In addition to the problems discussed above, reliable DTX detection becomes much more complicated when two orthogonal users (sub-channel-1 and 2) are dynamically going in and out of DTX mode and when the respective modulation is changing to and from AQPSK (two users active), GMSK (one user active), or a noise-like signal (no user active). In this scenario, the MS has to quickly detect that a DTX scenario has happened and respond accordingly.
For example, consider that MS1 and MS2 are two mobile stations paired for VAMOS. Further assume that MS1 is allocated more power than MS2. The network sets this power imbalance by using the ‘SCPIR’ value. So, the ‘SCPIR’ for MS1 is +ve. Now, in the paired condition, the modulation type is AQPSK, so MS1's receiver will detect the ‘SCPIR’ value as positive. (Generally, the MS employs an ‘alpha’ estimator to detect the ‘alpha’ value applied to the AQPSK signal. One technique for detecting a SCPIR value is described in US Patent Application Publication 2012/0244817, the entire contents of which are incorporated herein.) If the detected SCPIR is positive, then MS1 processes the received signal as is, e.g., using Single Antenna Interference Cancellation and/or other advanced receiver techniques, but generally without subtracting MS2's signal, i.e., the other sub-channel's data. MS2, on the other hand, will detect the ‘SCPIR’ as −ve, and will generally use one or more advanced receiver techniques, such as Successive Interference Cancellation (SIC) and/or joint detection (JD) to process and decode the weaker signal targeted to MS2.
In this scenario, if one of the mobile stations goes to DTX mode, then the modulation will switch from AQPSK to GMSK. The SCPIR value will also change. If earlier it was −ve, then it will go to +ve. This is what MS2 will perceive if MS1 goes to DTX mode. On the other hand, if MS2 goes to DTX, then MS1, which earlier observed a SCPIR value of +ve, will now observe an even more positive SCPIR.
As a result of these complexities, DTX detection based on SNR, Eb/No, or received signal power will not work in the VAMOS scenario. Accordingly, improved techniques are needed.